Environmental Requirements

Physical Attributes

High summer temperatures are unlikely to be a problem within British waters, but cold winter temperatures can have strong effects. Gruet (1982) reported that growth of S. alveolata is severely restricted below 5°C. Crisp (1964) noted that there were many severe losses of S. alveolata due to the severe winter of 1962-63, especially in South and North Wales, and in Lyme Bay where some colonies were depleted by half and others lost completely. Not surprisingly, survival was best at lower shore levels. Wilson (1971) found less severe effects at Duckpool in Cornwall, but nevertheless reported almost complete mortalities of some colonies, mainly at the higher shore levels. Further losses of reefs at Criccieth, North Wales were attributed to the cold winter of 1984 (Gubbay, 1988). Frost has been suggested as the factor limiting the upstream distribution of reefs in the Severn Estuary (Mettam, pers. comm. in Bamber & Irving, 1997). Growth rates increase with increasing temperature up to 20° C (Cunningham et al., 1984).

Reefs form mainly on the bottom third or so of the shoreline and in the shallow subtidal. Those in Duckpool, north east Cornwall were reported to be largely limited to areas exposed only by spring tides (Wilson, 1971), although they do penetrate as far up the shore as MLWN (Hawkins, pers. obs.). On Cumbrian shores hummocks forming at least 20% cover have been reported up to just below MHWN (Allen et al., 1991). The actual distribution on any one shore is probably related largely to substratum distribution and water movement.

True reefs have not often been reported to penetrate far subtidally, but this may to some extent reflect lack of survey data. Extensive shallow subtidal reefs have been found recently in the Severn Estuary (CCW, unpublished information), and in the same area they have been reported to penetrate as far as 20 m depth (Bamber & Irving, 1997). Quite widespread, although patchy, subtidal reefs were found off the Maryport - Dubmill Point area of the Cumbrian Coast by a series of grab surveys, and at Dubmill Point itself very extensive and dense hummocks could be seen extending beyond the level of a very large spring tide (Perkins, 1981). S. alveolata reefs are also reported to extend into the subtidal at Glassdrummand, Northern Ireland (Erwin et al., 1990) and also extended subtidally at Hilbre Island in the Dee Estuary in the early 1900’s (Herdman, 1919).

A supply of suspended coarse sediment is a requirement for the development of reefs, and the species has been reported to penetrate into areas such as the Severn Estuary where finer suspended sediments occur (Cunningham et al., 1984). Suspended sediment supply is affected by both the local availability of sediment and the amount of water movement for suspension (see d and e below).

It is widely reported that S. alveolata generally requires hard substrata on which to form, but that these must be in areas with a good supply of suspended coarse sediment. S. alveolata reefs can form on a range of substrata from pebble to bedrock (Cunningham et al., 1984). Reefs therefore commonly form on areas of rock or boulders surrounded by sand. Larsonneur (1984), working in the Bay of St Michel in Normandy, noted that the sand mason Lanice conchilega can stabilise sand well enough to allow subsequent colonisation by S. alveolata. Settlement occurs mainly on existing colonies or their dead remains (see chapter IV).

Water movement of sufficient intensity to suspend coarse sand particles, making them available for building the worms tubes, is a prime requirement. Cunningham et al. (1984) note that this may consist of waves or currents. In many British localities such as the south west of England, much of Wales and the Cumbrian coast the former seem more important, but in others such as parts of the Severn Estuary tidal suspension is probably very important. However, Sabellaria is generally absent from very exposed peninsulas such as the Lleyn, Pembrokeshire and the extreme south west of Cornwall, which probably relates to the effect of water movement on recruitment (Cunningham et al., 1984).

The complete absence of S. alveolata on the coast of the Isle of Man where there are apparently suitable shores seems odd. Cunningham et al. (1984) considered this as strong evidence of the importance of larval supply to the maintenance of populations (see chapter III). Larval supply itself may be strongly affected by local hydrography.

No detailed information on salinity requirements or tolerances have been found but the species does not normally seem to penetrate into very low salinity areas. However, Lancaster (1993) found extensive, healthy hummocks of Sabellaria overgrown with Fucus ceranoides - a fucoid normally found only in estuarine habitats - at Drigg, Cumbria, where there is a large freshwater input from the Drigg BNFL plant.

Larvae are strongly stimulated to settle by the presence of cement secretions of adult or juvenile Sabellaria and probably by other calcareous substrata (see chapter IV), though they will clearly also settle on non calcareous substrata.

Mussels Mytilus edulis and S. alveolata often occur together, and in many areas one or other may be dominant at different times. The factors affecting this interaction are not understood, though changes in sediment type and water movement are likely to be important factors which may tip the balance one way or another. It can be speculated that salinity and temperature changes could also play a part, since the tolerances of the two species to these are very different. Since both species have variable recruitment and depend upon very heavy recruitment to form dense beds, stochastic factors affecting larval recruitment will obviously also be important.

Wilson (1971) reports that the porches of the tubes are thin and easily damaged, but that the bulk of the colonies have a consistency resembling soft sandstone, and a considerable resistance to splitting. This seems perhaps to overstate the strength, however, given the experience of Cunningham et al. (1984) who carried out small scale experimental trampling: light trampling was found to be capable of causing obvious damage to the porches; they were able to inflict damage to a depth of up to 6 cm by using heavy pressure, particularly with the heel; and strong kicking caused structural damage to the whole colony, large cracks between the tubes, and the subsequent loss of substantial portion of one colony. Bait diggers appear to have little difficulty in breaking open colonies to collect the worms (Hawkins, pers. obs.). Loss of older colonies does occur during storms, perhaps aided by objects such as logs being tossed against them (Wilson, 1971), but over much of their range they are exposed at least occasionally to some fairly severe storm waves, which many of them clearly survive.

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